1. Field of the Invention
The present invention relates to frangible, ceramic conveyor rolls designed for use in the heat treatment of glass sheets wherein the rolls are rotated to transport a series of glass sheets through a hot enclosed atmosphere. The present invention also relates to a tunnel-like furnace provided with rolls of special construction according to the present invention.
Furnaces for tempering or annealing glass sheets conventionally include conveyors which carry the glass sheets through an enclosed atmosphere of high temperature during the annealing or tempering of the glass. A so-called roller hearth type of conveyor utilized to carry glass sheets includes horizontally extending conveyor rolls of ceramic material upon which heated glass sheets are rotatably supported. When the conveyor rolls rotate, the glass sheets are driven by friction over the rolls through the enclosed furnace atmosphere of high temperature.
One ceramic material used for cylindrically shaped rolls for a roller hearth is a sintered, fused silica material that is made by grinding solid fused silica into small particles and then sintering these particles to each other at a temperature below the fusing temperature. Such a material is readily formed into an elongated shape required for the rolls of the roller hearth. In addition, fused silica rolls so formed have a relatively small coefficient of thermal expansion. This latter characteristic is extremely desirable because the rolls are subjected to extremely high temperatures in one portion of the roller hearth and relatively low temperatures in another portion of the roller hearth.
One conventional way for rotatively supporting and driving conveyor rolls of fused silica and other like ceramic compositions utilizes bearings that support the opposite end portions of the rolls in the roller hearth. A drive member having a toothed drive component such as a gear or a chain sprocket drives the roll at one end. A metallic end cap rotatable with a driven shaft is conventionally used to transmit the drive from the drive gear or chain sprocket to the end of the ceramic roll.
Rolls of ceramic composition are superior to metal rolls in high temperature environments for several reasons. First of all, ceramic rolls have a relatively small coefficient of thermal expansion compared to that of metal rolls. Consequently, ceramic rolls are less likely to warp in response to a change of temperature. Ceramic rolls are also less likely to oxidize, flake off and develop pock marking on their glass engaging surface. However, ceramic materials are difficult to work with and are quite fragile. Furthermore, such rolls composed of ceramic material cannot be connected directly to the drive mechanisms needed to rotate the rolls. The use of metal end caps with ceramic rolls for roller hearths has been developed to a certain extent. However, they introduced problems of securing the metallic end caps to the ends of the ceramic rolls that they drive in transmitting a torque from the driving mechanism.
One method of securing involves an adhesive bond between the metal end cap and the end of the ceramic roll. Another method involves shrinking the metallic end cap onto the roll end by heat shrinkage. Still another method involves the use of O-rings of elastomeric material such as natural or synthetic rubber received within a circumferential groove extending around the inner surface of the cylindrical metal end cap. The O-ring is received within the groove in position to provide a frictional force sufficient to transmit a torque from the metal driving shaft through the cylindrically shaped metal end cap to the ceramic roll without undue wear of the roll, the O-ring or the metal cylindrical end cap. It has also been suggested to use a composite metal end cap in conjunction with a ceramic roll which comprises a flexible metal inner end cap member adapted for mounting in direct contact over an end of the ceramic roll and a rigid metal outer end cap member adapted for mounting over the inner end cap member and means for moving the outer end cap member axially relative to the inner end cap member so as to increase radial clamping of the flexible metal inner end cap member against the ceramic roll on axially inward motion of the rigid, outer end cap member.
All of these method have drawbacks. Adhesives presently known require an extended cure period of at least about 24 hours before the ceramic roll is securely fixed to its end cap. Such a delay is too long to accept for mass production operations. Furthermore, it is difficult to separate a metal end cap from the end of a ceramic roll that is adhered thereto when roll replacement is necessary.
Heat shrinking requires heating the metallic end cap to a very high temperature so that its diameter increases to enable it to be shrunk fit over the end of the ceramic roll. The internal diameter of the end cap and the outer diameter of the ceramic roll must be closely matched so that the end of the roll is small enough to be inserted into the heated end cap but large enough to be securely clamped after cooling. Often it is necessary to machine one or both of the matching parts in order to provide suitable matching. At the inner end of the metallic end cap, the ceramic roll is subjected to a clamping pressure due to the heat shrinking of the metal end cap. Adjacent to this portion of the ceramic roll subjected to the clamping pressure is an adjacent portion free of the clamping pressure. At this location of the boundary between a high clamping pressure and low clamping pressure, ceramic rolls are subject to fracture due to the high stress gradients involved.
The use of O-rings requires that the metal end caps be grooved circumferentially to receive the O-rings. Such machining is expensive. Furthermore, while it is possible to use ceramic rolls having O-rings providing frictional engagement with a metal end cap exterior to the furnace wall, the upper temperature limits to which the rolls and metal end caps can be exposed are limited to the temperature at which the material of which the O-rings are composed can withstand. While these temperatures are relatively high, further improvement would be desirable.
Making the metal end caps of the composite construction of a rigid outer member and a flexible machined inner member requires the use of several members for each end cap, each of which members have to be machined separately. It would be much better to develop a construction that utilizes a metal end cap of a construction that does not require complicated machining.
2. Description of Patents of Interest
U.S. Pat. No. 3,867,748 to Miller discloses the use of an adhesive for securing metal end caps to the ends of cylindrical ceramic conveying rollers in a roller hearth. The rolls cannot be used until the adhesive is cured. Since curing is a slow process, such rolls are not practical for rapid installation for high speed production operations. Furthermore, the end caps are vented to provide escape for excess adhesive, thereby complicating the end cap fabrication process.
U.S. Pat. No. 4,131,420 to Miller utilizes a two piece coupling that disconnects a ceramic roll including its end cap from the drive train which provides a rotating force to the end cap of the ceramic roll. The end cap in this patented apparatus is attached to the ceramic roll using an adhesive in a manner similar to that of the previous patent.
U.S. Pat. No. 4,140,486 to Nitschke utilizes a metallic spring end cap for rotatably driving a glass conveyor roll of ceramic material using helical spring coils that have a diameter in an undeflected condition slightly less than the diameter of the end of the ceramic roll so that coil deflection that increases the coil diameter allows the coils to be mounted over the end of the ceramic roll. A subsequent release clamps the coils over the end of the roll. The helical spring coil may be engaged by friction or by tooth sprocket gear driven rolls to transmit a force thereto.
U.S. Pat. No. 4,242,782 to Hanneken and Marriott discloses a ceramic roll for transporting glass sheets for thermal treatment in which a metal end cap is circumferentially grooved to receive a hard rubber O-ring that provides frictional engagement between the metal end cap and an end of the ceramic roll. While the metal end cap is constructed and arranged to facilitate its mounting on and removal from the end of the ceramic roll, it would be desirable to avoid the need for circumferentially grooving a cylindrical metal end cap.
U.S. Pat. No. 4,247,000 to Marriott and Imler discloses a composite metal end cap for use with a ceramic roll in which the end cap comprises a flexible metal inner end cap member adapted for mounting in direct contact over an end of the ceramic roll and a rigid metal outer end cap member adapted for mounting over the inner end cap member. The inner end cap member is constructed and arranged to flex radially inwardly in at least its axially inner end to provide a clamping force against the end of the ceramic roll when the outer member moves axially inwardly. It would be desirable to provide a metal end cap construction that is less complicated in structure.